[SPIR-V] Some local variables are missing information with -fspv-debug=vulkan-with-source

[sajjadmirzanv]

Description
In the linked Compiler Explorer example, there are 7 DebugLocalVariable instructions in 1.8.2505, but only 2 in trunk. "skyDot" (%23) for example has been deleted.

Ideally, even when variables are optimized away, DXC would keep the DebugLocalVariable instruction itself alive to represent that the variable did exist in the original program.

Steps to Reproduce
https://godbolt.org/z/cezPMaEEG

Actual Behavior
5 DebugLocalVariable instructions are missing.

Environment

• DXC version dxcompiler.dll: 1.9 - 1.8.2505.32 (b106a96); dxil.dll: 1.9(1.8.2505.32)
• Host Operating System Windows 10

[SteveUrquhart]

@s-perron, you can assign this to me, but it is an optimizer bug, not dxc.